FIELD OF THE INVENTION
The present invention relates generally to prosthetic devices for the replacement of vertebrae to treat patients having spinal resection, and more particularly to an improved adjustable, easily implantable vertebral body prosthesis for insertion and permanent installation intermediate the endplates of upper and lower vertebrae to replace the removed vertebral body and restore stability and normal vertebrae spacing to the spinal column while facilitating the occurrence of bony integration to fuse the aforesaid vertebrae together.
The main structural support of the human skeleton is the spinal column, a bony column that consists of a plurality of vertebrae which are interlinked by flexible joints, spaced apart by gelatinous intervertebral disks of fibrocartilage, and held together by ligaments. Each vertebra has a roughly cylindrical body, with wing-like projections, and a bony arch. The arches, which are positioned next to one another, create a tunnel-like space which houses the spinal cord. The anterior cylindrical bodies of the vertebrae, which are spaced apart by intervertebral disks, bear most of the compressive load of the spinal column (approximately 80 percent of the total load).
When it occurs, severe back pain can be among the most relentless and debilitating afflictions occurring to individuals, often making a normal life substantially impossible for victims of such conditions. The most common causes of severe spinal ailments include primary and metastatic malignant tumors which are unresponsive to standard therapy, non-malignant tumorous vertebrae, spinal cord compression associated with paresis or paraplegia, and vertebral collapse or backbone instability. These conditions all affect the anterior cylindrical body of a vertebra, which, as mentioned above, is the primary load-carrying part of the vertebrae.
The primary objectives of surgical intervention are to preserve the neurological function of the spinal cord and to relieve the intense pain associated with such conditions. It will be appreciated particularly by those skilled in the art that any such surgical intervention will necessarily involve the resection of the spinal column and the removal of the anterior cylindrical body of the vertebra. The resulting loss of bony support destabilizes the vertebral column, and therefore requires that the excised support material be replaced either by a prosthetic implant or other filler material.
One approach has been to remove the tumorous material, and then fill the space of the resected anterior spine with methylmethacrylate or some other plastic material. This approach has been less than successful, since it is difficult to achieve proper bonding with the bony material of the vertebrae. In addition, such materials often involve an exothermic chemical reaction for the polymerization of the plastic material, which can release a significant amount of heat into the adjacent tissue. In addition, these plastic materials do not exhibit sufficient mechanical strength and stability, even when they are reinforced with metal pins or struts.
Another approach which has been utilized is to use a hollow cylindrical mesh cage which is filled with bone chips or marrow. The bone material may be bone excised from the patient's own fibula or pelvis, or, alternately, allograft material, which is bone which typically has been harvested from a deceased donor. In the case of a metastatic tumor, bone cement may be used instead of bone chips or marrow.
A spreader is used to separate the vertebrae between which the cylindrical mesh cage is to be inserted. With the distance between the vertebrae maintained by the spreader, the cylindrical mesh cage is inserted into place, with the ends of the cylindrical mesh cage (which may include teeth) bearing on the opposing endplates of the vertebrae. The spreader is then released, so that normal compressive forces of the spine acting on the anterior column may anchor the cylindrical mesh cage in place. Bone cement nay also be applied at the ends of the cylindrical mesh cage to facilitate the ends of the cylindrical mesh cage being maintained in place.
Immediate stabilization of the spine following this procedure does not occur, since it generally takes between three and six months for bony fusion to take place. In addition, if the patient is to be treated by radiation and/or chemotherapy following the surgery, in many cases the radiation and/or chemotherapy will have an adverse affect on the bone graft, preventing it from surviving and fusing the two vertebrae together. In this case, additional surgery will generally be required to establish a satisfactory degree of spinal stability.
Another technique used to stabilize the spine following the removal of the anterior column of a vertebra is the use of a plurality of metal rods which are attached by bolts or screws to the two vertebrae on either side of the removed vertebrae. This technique presents a variety of problems, particularly due to the presence of large localized forces in the areas in which the rods are attached to the vertebrae by the bolts or screws. In addition, some areas of the spine are difficult or impossible to stabilize with this technique due to the presence of sensitive tissue located adjacent to the areas in which the stabilizing rods would be used.
It is accordingly the primary objective of the present invention that it provide an improved vertebral body prosthesis which may be used following the removal of the anterior column of a vertebra to reestablish spinal stability and maintain proper spacing between the vertebrae located immediately above and below the removed vertebra. It is an objective of the vertebral body prosthesis of the present invention that it be of a design and physical configuration which may be easily installed in place intermediate the endplates of the two adjacent vertebrae via a posterior surgical approach. It is a related objective of the vertebral body prosthesis of the present invention that the implant procedure not require the use of complex tools to install and position the vertebral body prosthesis intermediate the two vertebrae.
It is an further objective of the vertebral body prosthesis of the present invention that it be implantable in a surgical procedure reducing both the trauma to the patient and the time for the surgeon to implant the device. It is also an objective of the vertebral body prosthesis of the present invention that, when installed, it will securely and permanently maintain the integrity and security of the spinal column. It is yet another objective of the vertebral body prosthesis of the present invention that it promote prompt and permanent ingrowth of bone material intermediate the vertebrae located immediately above and below the removed vertebra to facilitate permanent fusion of the spinal segment. Still further objectives of the vertebral body prosthesis of the present invention are that it be made of biocompatible material compatible with long term implant in the human body, and that it be either adjustable in length or available in different sizes and configurations to fit a wide variety of patients and different locations in the spine.
The vertebral body prosthesis of the present invention must be of a construction which is both durable and long lasting, and it must require no maintenance once it is implanted. In order to enhance the market appeal of the vertebral body prosthesis of the present invention, it should also be of a simple mechanical design and relatively inexpensive construction to thereby afford it the broadest possible market. Finally, it is also an objective that all of the aforesaid advantages and objectives of the vertebral body prosthesis of the present invention be achieved without incurring any substantial relative disadvantage.